In trying to understand
pain and temperature, temperature is the easier sense. So, let's start there!
Either way, though, these senses are handled by free nerve ending sensory receptors.

Sensation of temperature:

You have already learned (back in our discussion of skin) that free
nerve endings detect temperature. But, it turns out that some free nerve endings can
detect heat, while other free nerve endings detect cold. Heat receptors tend to
detect temperatures over 25 °C (77 °F); they do not typically, however, continue to
detect heat when it gets too hot... like extremely hot and harmful temperatures of 45 °C
(113 °F) or hotter. Likewise, cold receptors respond to cool temperatures between
10 °C (50 °F) and 20 °C (68 °F), but they do not respond to really cold temperatures
below 10 °C.

We do feel the extremely hot or extremely cold sensations,
however... we just don't feel them as hot or cold-- just as pain! Our pain receptors
kick in at these extreme temperatures to warn us of potential harm.

Our temperature receptors tend to adapt, so that you can get
"used" to a temperature over time.

Sensation of pain:

We have many different stimuli that we consider painful, for
example:

Sharp mechanical stimulation (e.g., by a pin or
knife edge)

extremes of temperature

chemical pain (e.g., lactic acid build-up in
muscles)

Some of the pain receptors that we have
can detect all of these types of stimuli, while others are more specialized for just one
type.

Pain can be sensed through the skin, like temperature, touch and
pressure. However, pain can also be sensed in our viscera. Poor blood flow or
digestion can trigger pain receptors in the viscera and then we feel deeper pains.
This is called visceral pain.

The sensation of pain can be acute or chronic.
Acute pain is a sharp, unsustained pain. Chronic pain doesn't seem to go
away. Different pain sensory receptors cause acute pain from those that cause
chronic pain.

Referred Pain

A very interesting and clinically useful type of pain is called referred
pain. You may have heard that before a heart attack, many patients
experience a pain in their left chest, shoulder, or upper arm. A patient will tell
you that the pain they experienced in their arm, for example, before a heart attack was a
severely painful type, clearly felt within the arm, not the heart. It is a bizarre
thing, but we can't feel pain within our hearts, we just can't. Yet our bodies relay
(or refer) this pain to another location-- the arm in this case, even though the pain
isn't really from the arm.

How does this work? It is thought that as the pain sensory
neurons from viscera enter the spinal cord to synapse on their postsynaptic targets, they
overlap with pain sensory neurons coming in from the skin. The postsynaptic targets
of visceral and skin pain sensory neurons end up getting mixed input-- input from both
viscera and skin. Therefore, when the heart pain sensory neurons enter the spinal
cord, they, by accident, synapse on postsynaptic targets of the skin pain sensory neurons
from the left arm. Your brain only knows that spinal cord neurons that carry arm
pain information have been activated, so you feel arm pain. What a trick!

The good thing about this "mistake" is that it provides us
with a clinical warning. If we feel a bizarre pain, we may stop a strenuous activity
in time to not actually have a heart attack, but know enough to go to a doctor
immediately.

Referred pain not only informs us about the heart, but also about
other viscera, as shown in Figure 12.3.

Regulation of Pain

Our nervous systems have ways to regulate our pain normally,
although the nervous system of a person with real chronic pain does not seem to be able to
do this. Basically, when pain receptors are active for a while, their input affects
higher brain centers (through the spinothalamic tracts). These brain areas not only
tell our cortex to perceive pain in a certain region of the body, but they also send
descending information to our spinal cord to shut off the pain input.

Our spinal cord neurons respond to certain chemicals to shut off
their recognition of pain receptor input. These chemicals are neurotransmitters that
are released in the spinal cord, and include: enkephalins, serotonin, and
endorphins. Enkephalins are referred to as "opioids" because they work
much the same way as opium and morphine do, but they are naturally occurring within our
bodies.